This proposal describes a new imaging technology that can detect and track single RNA molecules in living cells in real time without limitation in observation time. The new technology is based on the distance dependent plasmon coupling between individual noble metal nanoparticles that are tethered by a RNA probe strand. Hybridization of a complementary RNA target strand induces an interparticle distance change that leads to a shift in the plasmon resonance wavelength. This spectral shift serves as fiduciary point indicating target RNA binding. The special novelty of the plasmon coupling approach is the active nature of the proposed nanoparticle sensors. Due to their inherent ability to couple with each other, the proposed sensors enable a continuous monitoring of the end-to-end distance of the RNA tether. The proposed sensors cannot only detect and track individual RNA molecules, they will also provide information about structural changes of the labeled RNA tether in real-time. The specific aims of the proposal are: Aim 1: Develop RNA coupled noble metal nanoparticle dimers for in vivo applications. Aim 2: Develop a plasmon coupling based detection technology for individual RNA molecules. Aim 3: Detect and track individual cytoplasmic RNA molecules in living cells in real time. The health benefits of the proposed research include the potential to develop effective ways to silence genes that cause serious diseases such as cancer. Other benefits to the biomedical community include new technologies that allow the detection of single RNA molecules in living cells thereby allowing an entirely new diagnostics with unknown sensitivity on the single cell level.